The present invention relates to a system and a method for calculating the quantity of intake air for controlling an engine for a motor vehicle.
In a fuel injection control system having an injection pulse width calculator based on the quantity of intake air, the intake air quantity must be measured with precision. As an intake air quantity sensor, an airflow meter with a hot film type or with a hot wire is provided in an intake passage at a position upstream of a throttle valve of the engine to detect the quantity of intake air.
Since the sensor has a high response, the output of the sensor oscillates as shown by the dot-dash line of FIG. 7, because of the pulsation of the intake air induced in the cylinders of the engine. Heretofore, the output Qs is averaged to obtain an intake air quantity Qs'.
In the fuel injection control system, a basic fuel injection pulse width Tp is determined in accordance with the intake air quantity Qs' and engine speed N as follows. EQU Tp=K.Qs'/N (K:constant)
An actual fuel injection pulse width Ti is obtained by correcting the basic fuel injection pulse width Tp with various coefficients such as a coolant temperature coefficient, an acceleration coefficient, and a feedback correcting coefficient, so that the air-fuel mixture is prevented from becoming rich or lean.
In an ignition timing control system, the basic fuel injection pulse width Tp obtained based on the quantity Qs' is regarded as engine load. An ignition timing is derived from an ignition timing map in accordance with the basic injection pulse width Tp and the engine speed N. The ignition timing is corrected with various coefficients corresponding to the above coefficients to determine an actual ignition timing.
When the throttle valve is rapidly opened for accelerating the engine, the intake air quantity sensor detects the amount of intake air Qs including the intake air induced in the cylinders of the engine and the intake air induced in an air chamber downstream of the throttle valve and an intake manifold. In other words, all the air passing through the throttle valve is measured by the sensor. Accordingly, the air actually induced in the cylinders can not be measured at once. The actual quantity induced in the cylinders appears on the output of the sensor with a delay D as shown in FIG. 7.
In a multiple-point injection system, injectors are disposed in terminal portions of an intake manifold. Since the injection time is set before the intake stroke of an engine, the air fuel mixture immediately after rapid opening of the throttle valve becomes lean for a moment. Then the quantity of fuel injection is determined based on the increased amount of the intake air. Thus, the air-fuel ratio rapidly becomes rich. As a result, amounts of HC and CO in the exhaust gas increase to make the emission poorer. Further, the power of the engine is temporarily reduced to lower the driveability of the motor vehicle. Similarly, when the throttle valve is rapidly closed, the air-fuel ratio deviates to make the emission poorer. The ignition timing is not also properly controlled at transient states.
Japanese Patent Application Laid-Open No. 59-200032 discloses a system where the basic injection pulse width Tp is corrected with a value based on the quantity of fuel injection calculated at the last injection to obtain a weighted means value.
However, in this system, the calculation is made at a predetermined crank angle, namely a calculating cycle .DELTA.t is determined in dependency on the engine speed N. Accordingly, the calculation cycle becomes long at a low speed range of the engine, so that the discrepancy of the fuel injection pulse width Ti with respect to the intake air quantity Qs becomes large.
If the calculating cycle .DELTA.t is set to meet to a low engine speed operation, the interval of the injection time becomes extremely short at a high engine speed range, so that an injection valve can not be properly controlled.
Japanese Patent Application Laid-Open No. 61-201857 discloses a system in which a calculating cycle .DELTA.t is determined in accordance with the engine speed. In the system, the actual intake air quantity actually induced in the cylinders is supposed to have a time lag of first order with respect to the time when the sensor detects the intake air quantity Qs. An estimated intake air quantity Q is calculated through the weighted means to synchronize the production of the quantity Q with the engine cycle. The intake air quantity Q(tn) at the present time is calculated by the following equation. EQU Q(tn)=(1-.alpha.)Q(tn-1)+.alpha.Qs (1)
where Q(tn-1) is the estimated intake air quantity at the last time, and
.alpha. is the weight for the weighted mean. The weight .alpha. is obtained by the following equation. ##EQU1## where .DELTA.t is the calculating cycle, and
.tau. is the time constant.
The time constant .tau. is obtained by the following equation. ##EQU2## where
a is a constant,
Vc is the capacity of an intake manifold,
VH is the total displacement of the engine,
N is the engine speed,
R is the gas constant, and
T is the absolute temperature.
However, it will be seen from the equations (2) and (3) that the weight .alpha. in the equation (1) is based on only the time constant .tau. dependent on the engine speed N. As shown in FIG. 8, in the range between t0 and t1 where the engine speed N does not rise (FIG. 8a) in accordance with the rapid opening of the throttle valve (FIG. 8b), the actual intake air quantity Q' increases with an increase of throttle valve opening degree .theta.. However, when the engine speed begins to rise, the calculated intake air quantity Q is delayed with the first order time lag (FIG. 8c) with respect to the increase of intake air quantity Q. As a result, the intake air quantity Q deviates from the actual intake air quantity Q' by a difference shown by the hatching.
Consequently, when the engine speed N rapidly rises at racing or a vehicle is started in a first speed of a transmission, the air-fuel ratio becomes temporarily lean. When the engine speed is rapidly reduced for changing the transmission ratio by closing the throttle valve, the air-fuel ratio becomes rich.
The actual quantity Q' varies in accordance with the return flow of the intake air at the overlapping period of an intake valve opening period and an exhaust valve opening period and with the throttle valve opening degree .theta.. However, the time constant .tau. does not compensate such a variation. Accordingly, the intake air quantity Q calculated based on the time constant .tau. can to deviate from a necessary quantity.
A microcomputer must take some time to calculate the weight .alpha. as a function of the time constant .tau.. Consequently, the time necessary for calculating the intake air quantity Q and the injection pulse width Ti are shortened by the calculation times for the coefficient. Therefore, the engine can not be properly controlled. In order to overcome this defect, a microcomputer having a large capacity must be used, which increases the manufacturing cost.